usually mid. Jan. into Feb. is the mating season. It would be dabateable as to weather or not the mating season is a "better" time to call them or not. One big strike against that line of thinking is that by that time of winter, they have been called to, in some places, a lot. SO, the remaining coyotes that have survived the cold harsh winter conditions and all the hunting pressure, will be tougher to call.
With all that said, the mating season is prime time to use howling as a primary tool to call them in. they can still be called during late winter, a guy just needs to change tactics/sounds a little to insure success.
This is one reason that a lot of guys avoid any coyote vocals during the early winter time of calling. Kind of keeps an ace in the hole for late winter, a sound that hasn't been used against them.